Please help, (Drugs and alcohol)

I once had a really good friend for a long time and it was amazing at the beginning. Later in the friendship, she started abusing drugs. Hard drugs, lots of them. I would get yelled at and snapped at every day. She would consistently blame me for everything and then try and laugh it off because she knew she had burned all her other bridges and I was the only one she had left. I didn't want to leave her she was one of my best friends I wanted to help

Until every. Single. Day. I would get yelled at hourly and snapped at over and over and over again she was just always so grumpy and angry and in withdrawal. Eventually one day she told me I wasnt good enough for her anymore and she ditched me completely. That was a couple years ago. Now, at my job, which I have had for over a year, its happening AGAIN but this time with my co worker. The two of us only ever work together as that's how the schedule works and she used to be quite a lot like me actually hated drugs only drank once in a while, etc. Now every time I go to work I smell at LEAST weed. Shes owed me $200 for about half a year now. I had to lend her because she almost got herself hurt one time not being able to pay her dealer back on time and she makes up an excuse every weekend as to why she didnt any back to me but she will next time. Every day she talks about how she almost died last night because she drank too much and how she screwed guy after guy at her friend's house that shes always at. And every 5 minutes, I'm corrected, yelled at or snapped at for absolutely nothing as she sits on her ass and doesn't do her job at all. Ever. Our boss knows this and yells at her sometimes for it but not often not often at all and will never fire her because they've known each other forever and are like family. But I've had this friendship before. I cannot help her. I have no idea what to do. I don't necessarily want to quit but I will consider it because I'm not sure I can be a part of this kind of relationship again.